Field of the Invention
The present invention relates generally to low compression multi-piece golf balls having a solid core comprising at least one layer made of a plasticized thermoplastic composition. Preferably, the ball has a compression of less than 60 and more preferably less than 50. In one example, a two-piece ball containing an inner core (center) and protective outer cover may be made. In another example, a three-piece ball containing an intermediate layer disposed between the core and cover may be made. Preferably, the plasticized thermoplastic composition comprises an ethylene acid copolymer ionomer and plasticizer.
Brief Review of the Related Art
Multi-layered, golf balls are used today by recreational and professional golfers. In general, these golf balls contain an inner core protected by a cover. The core acts as the primary engine for the ball and the cover protects the core and helps provide the ball with durability and wear-resistance. The core and cover may be single or multi-layered. For example, two-piece golf balls containing an inner core that is protected by an outer cover are popular. Three-piece golf balls having an inner core, intermediate layer, and outer cover layer are used by many professional and recreational golfers. In other instances, golfers will use a four-piece ball containing a dual-core (inner core and surrounding outer-core layer) and dual-cover (inner cover layer and surrounding outer cover layer). Intermediate layer(s) may be disposed between the core and cover layers to impart various properties. Thus, five-piece and even six-piece balls can be made. Normally, the core layers are made of a natural or synthetic rubber material or an ionomer polymer. These ionomer polymers are typically copolymers of ethylene and methacrylic acid or acrylic acid that are partially or fully neutralized. In particular, highly neutralized polymer (HNP) compositions may be used to form a core layer. Metal ions such as sodium, lithium, zinc, and magnesium are commonly used to neutralize the acid groups in the acid copolymer.
Such ethylene acid copolymer ionomer resins generally have good durability, cut-resistance, and toughness. These ionomers may be used to make cover, intermediate, and core layers for the golf ball. When used as a core material, the ionomer resin helps impart higher initial velocity to the golf ball and this helps the ball have longer flight distance, which is particularly important for driver shots off a tee.
The durability, spin rate, and feel of the ball also are important properties. In general, the durability of the ball refers to the impact-resistance of the ball. Balls having low durability appear worn and damaged even when such balls are used only for brief time periods. In some instances, the cover may be cracked or torn. The spin rate refers to the ball's rate of rotation after it is hit by a club. Balls having a relatively high spin rate are advantageous for short distance shots made with irons and wedges. Professional and highly skilled amateur golfers can place a back spin more easily on such balls. This helps a player better control the ball and improves shot accuracy and placement. By placing the right amount of spin on the ball, the player can get the ball to stop precisely on the green or place a fade on the ball during approach shots. On the other hand, recreational players who cannot intentionally control the spin of the ball when hitting it with a club are less likely to use high spin balls. For such players, the ball can spin sideways more easily and drift far-off the course, especially if it is hooked or sliced. Meanwhile, the “feel” of the ball generally refers to the sensation that a player experiences when striking the ball with the club and it is a difficult property to quantify. Balls are often described as having a soft or hard feel. The feel of the ball primarily depends upon the hardness and compression of the ball.
In recent years, golf balls having a lower compression and spin rate have been developed. For example, three-piece balls containing an inner core and outer core layer (or intermediate layer) made of polybutadiene rubber and outer covers made of ionomer resin have been developed. These relatively low compression and low spin balls are particularly desirable when making short shots around the green, because the player senses more control with such balls and shot accuracy often is improved.
Golf balls having a relatively low compression are described in the patent literature. For example, Ogg et al., U.S. Pat. No. 7,918,748 discloses a two-piece golf ball comprising a core comprising made of a neodymium-catalyzed polybutadiene composition, and a cover made of partially-neutralized ethylene acid copolymer composition, wherein the golf ball has a diameter of at least 1.68 inches, a mass of at least 45 grams, a coefficient of restitution of at least 0.780, and a PGA compression ranging from 35 to 50. Lemons et al., U.S. Pat. No. 7,431,669 discloses a low compression golf ball, wherein the core is formed of polybutadiene rubber that produces a zero (or less) compression. The mantle is molded around the core using ethylene, acrylic acid and n-butyl acrylate terpolymers, with 100% of the acrylic acid groups neutralized with metal ions. The cover is molded using conventional ethylene acid copolymer ionomers. Cavallaro, U.S. Pat. No. 5,947,842 discloses a low-spin multi-layer golf ball containing a core, a cover layer, and a mantle layer disposed between the core and cover layer. The core preferably has a compression of less than 80 points and more preferably less than 60 points. According to the '842 Patent, calcium oxide can be added to the core composition in place of zinc oxide and this helps reduce core compression by at least about 2 compression points. Loper et al., US Patent Application Publication 2009/0170634 discloses a golf ball comprising: (a) a core; (b) an inner mantle layer; (c) an intermediate mantle layer; (d) an outer mantle layer; and (e) at least one cover layer; wherein the mantle layers may be made of various ionomer compositions, and the core has a PGA compression of less than 70, and the combined sub-assembly of core/inner mantle layer/intermediate mantle layer has a PGA compression of at least 30.
Although some of the above-described compositions may be somewhat effective for making certain components and layers in a multi-piece golf ball, there is still a need for new compositions that can impart high performance properties to the ball. One problem with some conventional low compression balls is they may have low impact durability. The balls may appear excessively worn and small cracks may appear after repeated use. In particular, there is a continuing need for improved core constructions in golf balls. The core material should help provide the ball with a relatively low compression and soft feel. The ball should have good spin control. At the same time, the impact durability and resiliency (as measured by Coefficient of Restitution) properties of the ball should not be sacrificed. The ball should have high impact durability and resiliency. The present invention provides golf balls having an optimum combination of properties.